1. Field of the Invention
The present invention relates to a method for forming a metal silicide layer, and more particularly to a method for forming a cobalt silicide layer or a titanium silicide layer.
2. Description of Related Art
Semiconductor devices have been becoming smaller and smaller in recent years, and this has been accompanied by increases in gate metallization resistance, and in parasitic resistance or contact resistance of the source and drain portions of transistors. Consequently, it has been very difficult to realize the higher speeds that would be anticipated from the scaling rule. A silicide technique, in which a silicide film of a high-melting-point metal is formed in self-aligning fashion over gate and diffusion layer regions, has come to the forefront as a way to solve this problem. This silicide technique affords a decrease in sheet resistance. The use of titanium silicide (TiSi2) or cobalt silicide (CoSi2) is especially promising from the standpoints of lowering the resistance and achieving thermal stability.
When TiSi2 is used in a fine device having fine wiring on the order of 0.1 xcexcm, however, it is necessary during the step of forming the TiSi2 to effect a phase transformation from high resistance C49 phase TiSi2 (hereinafter referred to as C49-TiSi2) to low resistance C54 phase TiSi2 (hereinafter referred to as C54-TiSi2). With finely patterned C49-TiSi2, however, this phase transformation is restricted, which creates a problem called the narrow line effect, in which it is difficult to lower the resistance. If an attempt is made to make the wiring even finer (to less than 0.1 xcexcm), there is a sharp increase in the sheet resistance of TiSi2. This is attributable to the aggregation of the TiSi2 and the resulting disconnection of the silicide layer.
In contrast, the narrow line effect is not encountered when CoSi2 is used. Also, in the formation of the CoSi2, a cap film made from titanium nitride (TiN) or titanium (Ti) is provided over the top layer of cobalt (Co) provided over the silicon (Si). This allows the oxidation of the surface of the cobalt layer to be suppressed. According to published literature (1993 IEDM Tech. Dig., p. 906), it is possible to lower the resistance of a narrow line pattern of 0.075 xcexcM CoSi2, for example.
Let us at this point refer to FIGS. 7A-7D to give a brief description of a common method for using a suicide technique to form a low-resistance CoSi2 layer over a polysilicon gate electrode and a diffusion layer. FIGS. 7A-7D are simplified process diagrams of a conventional low-resistance CoSi2 layer, and illustrates the steps with cross sections of the structure during formation.
First, a polysilicon gate electrode 102 and diffusion layers 104 that flank this gate electrode 102 on both sides are formed on a silicon substrate 100 by an ordinary method. Isolation regions 106 are on the outsides of the diffusion layers 104, formed from thick oxide films. Side wall oxide films 108 are formed on the side walls of the gate electrode 102. Next, a cobalt film 110 is formed by sputtering over the entire main surface of the substrate 100 on which the gate electrode 102 and the diffusion layers 104 have been formed (FIG. 7(A)). After this, an RTA (Rapid Thermal Annealing) treatment (xe2x80x9cfirst RTA treatmentxe2x80x9d) is performed at a temperature of 450 to 650xc2x0 C. The result of this is that the surface of the gate electrode 102 reacts with the portion of the cobalt film 110 in contact with this surface, forming a first CoSi layer 112 (a layer made up of CoSi, CoSi2, and a compound of CoSi and CoSi2). The surface of the diffusion layers 104 also reacts with the portion of the cobalt film 110 in contact with this surface, forming a second CoSi layer 114 (a layer made up of CoSi, CoSi2, and a compound of CoSi and COSi2) (FIG. 7(B)) After this, a sulfuric acid peraqueous solution, a hydrochloric acid peraqueous solution, or the like is used to selectively remove a portion of the unreacted cobalt film 110 (FIG. 7(C)). A second RTA treatment is then performed at a temperature of 750 to 900xc2x0 C., which changes the first CoSi layer 112 and the second CoSi layer 114 into first and second CoSi2 layers 116 and 118 (layers made up almost entirely of CoSi2). Thus, a low-resistance first CoSi2 layer 116 and second CoSi2 layer 118 can be formed on the surface of the gate electrode 102 and on the diffusion layers 104 (FIG. 7(D)).
However, the junction depth of a device tends to be shallow as the device becomes more highly integrated. Accordingly, if too much silicon is consumed in the step in which the silicon on the surface of the diffusion layers 104 reacts with the cobalt film 110 to form a CoSi layer, there is the danger that this junction will be broken and leakage current will be generated. It is therefore necessary to reduce the amount of silicon consumption.
One way to reduce the amount of silicon consumption is to form a thinner cobalt film. However, when a cobalt film with a thickness of 10 nm, for example, is formed by sputtering as in the past, the sputtering only lasts about 13 seconds. Therefore, if an attempt is made to make the film even thinner, the film formation time will be shorter still, making it difficult to control the film thickness.
It is an object of the present invention to provide a method with which a metal silicide layer with low resistance can be easily formed on a substrate, with minimal consumption of silicon.
In order to achieve this object, the method of the present invention for forming a metal silicide layer comprises the following steps in the formation of a metal silicide layer over a silicon-containing region.
(1-1) A step of forming a first metal layer by depositing a first metal over the silicon-containing region.
(1-2) A step of forming a second metal layer by depositing a second metal over the first metal layer.
(1-3) A step of forming an antioxidation layer over the second metal layer.
(2) A step of performing a first heat treatment on the structure comprising the silicon-containing region, the first metal layer, the second metal layer, and the antioxidation layer (those formed by deposition in steps (1-1) to (1-3)), thereby:
(a) forming a metal silicide preliminary layer from a region on the first metal layer side of the silicon-containing region and a region on the silicon-containing region side of the first metal layer, and
(b) forming an alloy layer including a first metal and a second metal from the second metal layer and a region on the second metal layer side of the first metal layer.
(3) A step of removing the antioxidation layer and then removing the alloy layer and a portion of the remaining first metal layer.
(4) A step of performing a second heat treatment on the metal silicide preliminary layer (the layer formed in step (2)) at a higher temperature than in the first heat treatment so as to change the metal silicide preliminary layer into a metal silicide layer.
With the present invention, the above-mentioned step (2), in which the first heat treatment is performed, involves forming an alloy layer including a first metal and a second metal from the second metal layer and a region on the second metal layer side of the first metal layer, so the metal silicide preliminary layer formed from the region on the first metal layer side of the silicon-containing region and the region on the silicon-containing region side of the first metal layer can be formed in a thickness of only 35 nm or less. This is because part of the first metal layer is consumed in the formation of the alloy layer, so the region of the first metal layer consumed in the formation of the metal silicide preliminary layer is smaller (narrower) than in the past. The above-mentioned step (4), in which the second heat treatment is performed on this metal silicide preliminary layer, makes it possible for a thin-film metal silicide layer to be formed over the silicon-containing region. As a result, if a metal silicide layer is formed over the surface of the diffusion layer of a semiconductor device using this method, the amount of silicon consumed on the surface of the diffusion layer will be smaller than in the past, so there is no danger of junction breakage even if the diffusion layer has a shallow junction.
When a cobalt silicide layer is formed as the metal silicide layer, it is preferable for the first metal to be cobalt, and for the second metal to be titanium.
Using these metals in the above-mentioned step (1) will yield a structure including a cobalt layer as the first metal layer over the silicon-containing region, a titanium layer as the second metal layer over this cobalt layer, and an antioxidation layer over this titanium layer. In the above-mentioned step (2), a cobalt silicide preliminary layer (CoSi layer) is formed over the silicon-containing region, and an alloy layer whose main components are cobalt and titanium (Coxe2x80x94Ti(Si) alloy layer) can be formed over the cobalt silicide preliminary layer. It is possible that this alloy layer will contain a small amount of silicon. The antioxidation layer is then removed, and the cobalt layer and alloy layer are removed, after which the second heat treatment is performed to change the CoSi layer into a low-resistance CoSi2 layer, forming a thin-film cobalt silicide layer (CoSi2 layer) over the silicon-containing region. Thus, the use of the method of the present invention makes it possible to form a cobalt silicide layer that is a thin film and has low resistance over the surface of a gate electrode or diffusion layer in a fine semiconductor device having fine wiring on the order of 0.1 xcexcm.
When a titanium silicide layer is formed as the metal silicide layer, it is preferable for the first metal to be titanium, and for the second metal to be cobalt.
Using these metals in the above-mentioned step (1) will yield a structure including a titanium layer as the first metal layer over the silicon-containing region, a cobalt layer as the second metal layer over this titanium layer, and an antioxidation layer over this cobalt layer. In the above-mentioned step (2), a titanium silicide preliminary layer (C49-TiSi2 layer) is formed over the silicon-containing region, and a Coxe2x80x94Ti(Si) alloy layer is formed over the titanium silicide preliminary layer. The antioxidation layer is then removed, and the titanium layer and alloy layer are removed, after which the second heat treatment is performed to change the C49-TiSi2 layer into a low-resistance C54-TiSi2 layer, forming a thin-film (thickness of 35 nm or less) titanium silicide layer (C54-TiSi2 layer) over the silicon-containing region.
It is preferable for the antioxidation layer to be a titanium nitride layer or a tungsten layer.
Any semiconductor material able to withstand the temperature of the first heat treatment can be used as the material that makes up the antioxidation layer. Examples include TiN (titanium nitride), Ta (tantalum), TaN (tantalum nitride), Ru (ruthenium), Ni (nickel), Cu (copper), Mo (molybdenum), W (tungsten), and other heat-resistant materials. Of these, it is preferable to use a material that can be easily made into a film by sputtering, that lends itself well to selective etching, and that has for some time been readily available as a semiconductor material. With these aspects in mind, the use of TiN (titanium nitride) or tungsten (W) is preferred.
It is preferable for the first heat treatment to be performed within a temperature range of 450 to 750xc2x0 C. (not including 750xc2x0 C.), and for the second heat treatment to be performed within a temperature range of 750 and 900xc2x0 C.
The reaction between the silicon and the first metal can be conducted selectively by performing the first heat treatment within a temperature range of 450 to 750xc2x0 C. Also, the high-resistance metal silicide preliminary layer obtained by reaction between the silicon and the first metal can be changed into a low-resistance metal silicide layer by performing the second heat treatment within a temperature range of 750 to 900xc2x0 C.
Preferably, when the first metal is cobalt and the second metal is titanium, the first heat treatment is performed within a temperature range of 450 to 650xc2x0 C. When the first metal is titanium and the second metal is cobalt, it is preferable for the first heat treatment to be performed within a temperature range of 600 to 750xc2x0 C.
In the simultaneous formation of metal silicide layers of different thickness over different silicon-containing regions, it is preferable for the following steps to be included. Here, a first silicon-containing region and a second silicon-containing region are provided as the different silicon-containing regions, and a thick metal silicide layer is provided over the second silicon-containing region.
(1-1) A step of forming first metal layers by depositing a first metal over a first silicon-containing region and a second silicon-containing region.
(1-2) A step of forming a second metal layer by depositing a second metal over the first metal layer.
(1-2#) A step of removing part of the second metal layer located over the second silicon-containing region.
(1-3) A step of forming an antioxidation layer over the remaining second metal layer and over the first metal layer exposed from the second metal layer.
(2) A step of performing a first heat treatment on the structure formed through steps (1-1) to (1-3), thereby:
(a)-1: forming a metal silicide first preliminary layer from a region on the first metal layer side of the first silicon-containing region and a region on the first silicon-containing region side of the first metal layer,
(a)-2: forming a metal silicide second preliminary layer that is thicker than the first preliminary layer from a region on the first metal layer side of the second silicon-containing region and a region on the second silicon-containing region side of the first metal layer, and
(b) forming an alloy layer including a first metal and a second metal from a region on the second metal layer side of the first metal layer and the remaining second metal layer.
(3) A step of removing the antioxidation layer and then removing the alloy layer and a portion of the remaining first metal layer.
(4) A step of performing a second heat treatment on the first preliminary layer and second preliminary layer at a higher temperature than in the first heat treatment so as to change the first preliminary layer and second preliminary layer into a metal silicide first layer and a metal silicide second layer.
As a result, a first layer of thin-film metal silicide can be formed over the first silicon-containing region, and a second layer of metal silicide that is thicker than the first layer can be simultaneously formed over the second silicon-containing region. This is because the second metal layer was removed from over the second silicon-containing region in step (1-2#), so the alloy layer including the first metal layer and second metal layer was not formed. Because no alloy layer is formed, the first metal layer consumes more of the silicon in the second silicon-containing region, allowing a thicker metal silicide layer to be formed. Thus, the thickness of the metal silicide layer can be varied as required. When this method is employed to form a semiconductor device, a thin film of adequate thickness for the junction not to be broken can be formed over a diffusion layer with a shallow junction, for example. With a gate electrode with which there is no worry about junction breakage, on the other hand, a thicker metal silicide layer can be formed in an effort to further lower the resistance.
Also, with this method for forming a metal silicide, when the metal silicide layer is a cobalt silicide layer, it is preferable for the first metal to be cobalt, and for the second metal to be titanium. When the metal silicide layer is a titanium silicide layer, it is preferable for the first metal to be titanium, and for the second metal to be cobalt.
It is also preferable for the antioxidation layer to be a titanium nitride layer or a tungsten layer.
It is preferable for the first heat treatment to be performed within a temperature range of 450 to 750xc2x0 C. (not including 750xc2x0 C.), and for the second heat treatment to be performed within a temperature range of 750 and 900xc2x0 C.
Also, the antioxidation layer is removed in the above-mentioned method for forming a metal silicide layer, and when a TiN layer, for example, is used as the antioxidation layer, it is preferable to remove it by wet etching using an ammonia peraqueous solution (a mixture of H2O, NH4OH, and H2O2). Doing so allows the TiN layer to be selectively removed.
Also, the alloy layer and a portion of the remaining unreacted first metal layer are removed after the antioxidation layer has been removed, and when cobalt is used as the first metal, both the first metal layer and the alloy layer can be removed by wet etching using a sulfuric acid peraqueous solution (a mixture of H2SO4 and H2O2) or a hydrochloric acid peraqueous solution (a mixture of H2O, HCl, and H2O2). When titanium is used as the first metal, it is preferable for the alloy layer to be removed with a sulfuric acid peraqueous solution or a hydrochloric acid peraqueous solution, and for the first metal layer to be removed with an ammonia peraqueous solution.